1. Field of the Invention
The present invention relates in general to the field of signal processing, and, more specifically, to a switch-mode converter operating in a hybrid discontinuous conduction mode (DCM)/continuous conduction mode (CCM).
2. Description of the Related Art
Switch-mode converters are Direct Current (DC) to Direct Current (DC) converters that convert from one DC voltage level to another DC voltage level. A switch-mode converter operates by temporarily storing input energy at one voltage level and respectively releasing the energy at its output at a different voltage level. Two main exemplary switch-mode converters are a switch-mode boost converter and a switch-mode buck converter Both of these switch-mode converters are well known in the art.
An exemplary switch-mode boost converter/stage 102 is shown in a typical exemplary power factor corrector 100 in FIG. 1. Power factor correctors often utilize a switch-mode boost stage to convert alternating current (AC) voltages (such as line/mains voltages) to direct current (DC) voltages or DC-to-DC wherein the input current is proportional to the input voltage. Power factor correctors provide power factor corrected (PFC) and regulated output voltages to many devices that utilize a regulated output voltage. Switch-mode boost converter/stage 102 will now be explained in more detail in the context of power factor corrector 100.
Voltage source 101 supplies an alternating current (AC) input voltage Vin(t) to a full-wave diode bridge rectifier 103. The voltage source 101 (e.g., voltage Vin(t)) is, for example, a public utility, such as a 60 Hz/120 V line (mains) voltage in the United States of America or a 50 Hz/230 V line (mains) voltage in Europe. The input rate associated with input voltage Vin(t) is the frequency of voltage source 101 (e.g., 60 Hz in the U.S. and 50 Hz. in Europe). The rectifier 103 rectifies the input voltage Vin(t) and supplies a rectified, time-varying, line input voltage Vx(t) to the switch-mode boost stage 102. The actual voltage at any time t is referred to as the instantaneous input voltage. Unless otherwise stated, the term “line rate” is hereafter referred to and defined as the rectified input frequency associated with the rectified line voltage Vx(t). The line rate is also equal to twice the input frequency associated with input voltage Vin(t). The rectified line input voltage is measured and provided in terms of Root Mean Square (RMS) voltage, e.g., Vrms.
The switch-mode boost converter/stage 102 includes a switch 108 (e.g., Field Effect Transistor (FET)) by which it is controlled and provides power factor correction (PFC) in accordance with how switch 108 is controlled. The switch-mode boost converter/stage 102 is also controlled by switch 108 and regulates the transfer of energy from the rectified line input voltage Vx(t) through inductor 110 to capacitor 106 via a diode 111. The inductor current iL ramps ‘up’ when the switch 108 conducts, i.e. is “ON”. The inductor current it, ramps down when switch 108 is nonconductive, i.e. is “OFF”, and supplies current iL to recharge capacitor 106. The time period during which inductor current iL ramps down is commonly referred to as the “inductor flyback time”.
A switch-mode converter controller 114, such as an exemplary power factor correction (PFC) controller, controls switch 108. Switch-mode converter controller 114 controls switch 108 and, thus, controls power factor correction and regulates output power of the switch-mode boost converter/stage 102. The goal of power factor correction technology is to make the switch-mode boost converter/stage 102 appear resistive to the voltage source 101. Thus, the switch-mode converter controller 114 attempts to control the inductor current iL so that the average inductor current iL is linearly and directly related to the rectified line input voltage Vx(t). Unitrode Products Datasheet entitled “UCC2817, UCC2818, UCC3817, UCC3818 BiCMOS Power Factor Preregulator” (SLUS3951) dated February 2000-Revised February 2006 by Texas Instruments Incorporated, Copyright 2006-2007 (referred to herein as “Unitrode datasheet”) and International Rectifier Datasheet entitled “Datasheet No. PD60230 rev C IR1150(S(PbF) and IR 11501(S)(PbF)” dated Feb. 5, 2007 by International Rectifier, describe examples of a PFC controller. The PFC controller supplies a pulse width modulated (PWM) control signal CS0 to control the conductivity of switch 108.
Two modes of switching stage operation exist: Discontinuous Conduction Mode (“DCM”) and Continuous Conduction Mode (“CCM”). In DCM, switch 108 of switch-mode converter controller 114 (or boost converter) is turned on (e.g., “ON”) when the inductor current iL equals zero. In CCM, switch 108 of switch-mode converter controller 114 (or boost converter) switches “ON” when the inductor current is non-zero, and the current in the energy transfer inductor 110 never reaches zero during the switching cycle. In CCM, the current swing is less than in DCM, which results in lower I2R power losses and lower ripple current for inductor current iL which results in lower inductor core losses. The lower voltage swing also reduces Electro Magnetic Interference (EMI), and a smaller input filter can then be used. Since switch 108 is turned “OFF” when the inductor current iL is not equal to zero, diode 111 needs to be very fast in terms of reverse recovery in order to minimize losses.
The switching rate for switch 108 is typically operated in the range of 20 kHz to 100 kHz. Slower switching frequencies are avoided in order to avoid the human audio frequency range as well as avoid increasing the size of inductor 110. Faster switching frequencies are typically avoided since they increase the switching losses and are more difficult to use in terms of meeting Radio Frequency Interference (RFI) standards.
Capacitor 106 supplies stored energy to load 112. The capacitor 106 is sufficiently large so as to maintain a substantially constant link output voltage Vc(t) through the cycle of the line rate. The link output voltage Vc(t) remains substantially constant during constant load conditions. However, as load conditions change, the link output voltage Vc(t) changes. The switch-mode converter controller 114 responds to the changes in link output voltage Vc(t) and adjusts the control signal CS0 to resume a substantially constant output voltage as quickly as possible. The switch-mode converter controller 114 includes a small capacitor 115 to prevent any high frequency switching signals from the line (mains) input voltage Vin(t).
Switch-mode converter controller 114 receives two feedback signals, the rectified line input voltage Vx(t) and the link output voltage Vc(t), via a wide bandwidth current loop 116 and a slower voltage loop 118. The rectified line input voltage Vx(t) is sensed from node 120 between the diode rectifier 103 and inductor 110. The link output voltage Vc(t) is sensed from node 122 between diode 111 and load 112. The current loop 116 operates at a frequency fc that is sufficient to allow the switch-mode converter controller 114 to respond to changes in the rectified line input voltage Vx(t) and cause the inductor current iL to track the rectified line input voltage Vx(t) to provide power factor correction. The inductor current iL controlled by the current loop 116 has a control bandwidth of 5 kHz to 10 kHz. The voltage loop 118 operates at a much slower frequency control bandwidth of about 5 Hz to 20 Hz. By operating at 5 Hz to 20 Hz, the voltage loop 118 functions as a low pass filter to filter a harmonic ripple component of the link output voltage Vc(t).
FIG. 1B shows an exemplary switch-mode buck converter 150 that comprises the similar elements that were used for switch-mode boost converter/stage 102 in FIG. 1A. Switch-mode converter controller 114 is coupled to-switch-mode buck converter 150. Switch-mode converter controller 114 executes a switch control algorithm which defines switching characteristics for the switch control signal CS0 that is used to control switch 108.
Switch-mode buck converter 150 includes switch 108 coupled in series with inductor 110. One end of diode 111 is coupled between switch 108 and inductor 110 at the positive side of the input voltage Vin. The other end of diode 111 is coupled to the negative side of input voltage Vin. Capacitor 106 is coupled across the output voltage Vout. In contrast, for a switch-mode buck converter (e.g., switch-mode buck converter 150), the average inductor current is the output current of the buck converter, and the input current is approximately calculated as:Iin=Iout*Vout/Vin  Equation A
This mode of operation for the switch-mode buck converter requires the output voltage Vout to be less than the input voltage Vin. In some applications, the output current Iout is directly controlled, such as for LED lighting. In other applications, the output voltage Vout requires regulation, and current control is still desirable. In FIG. 1B, switch-mode converter controller 114 is coupled to switch-mode buck converter 150 in the manner shown. The current loop 152 operates at a frequency fc that is sufficient to allow the switch-mode converter controller 114 to respond to changes in the rectified line input voltage Vx(t). A voltage feedback loop 154 controls the input to a current regulator.
With reference now to FIG. 2A, a plot 200 of exemplary DCM current waveforms is shown for a switch control algorithm for controlling a switch (e.g., switch 108) of a switch-mode boost converter (e.g., switch-mode boost converter/stage 102) at a time scale of 10 microseconds wherein the target current itarget in FIG. 2A is set at 0.8 Amp. Plot 200 shows the current waveform for inductor current iL through inductor 110. Exemplary on-time ton and off-time toff are also shown. In this case, since the target current itarget is low and below the exemplary minimum target current itarget of 1 Amp for operating in CCM, the switch-mode boost converter/stage 102 operates in DCM.
With reference now to FIG. 2B, a plot 202 of exemplary DCM current waveforms is shown for a switch control algorithm for controlling a switch (e.g., switch 108) of a switch-mode boost converter (e.g., switch-mode boost converter/stage 102) at a time scale of 10 microseconds wherein the target current itarget in FIG. 2A is set at or very close to 1 Amp (e.g., the exemplary minimum target current level itarget). Plot 202 shows the current waveform for inductor current iL through inductor 110. Exemplary on-time ton and off-time toff are again shown. In this case, since the target current itarget is at or very close to the minimum target current itarget of 1 Amp for operating in DCM, the switch-mode boost converter/stage 102 is in a transitional conduction mode in which operation of the switch-mode boost-stage/converter 102 may be able to be switched to CCM.
Several advantages of operating the switch-mode converter (e.g., switch-mode boost converter 102) in CCM exist. For example, “shoot-through” conduction, in which the diode (e.g., diode 111) and the switch (e.g., switch 108) are both on for the same (transient) time, does not exist. The switch (e.g., switch 108) always turns on with zero current (other than for parasitics). These advantages allow for good switch-mode conversion efficiency at low cost. Also, the control of the switch for the switch-mode converter can be entirely open loop, with no need to sense the actual inductor current.
However, there are disadvantages for operating a switch-mode converter in CCM. One disadvantage is that high ripple in the inductor current (e.g., inductor current iL) exists. The switch-mode converter in CCM also has a limited power range. In CCM, the switch-mode converter has a peak current that is limited by the saturation limit of the inductor. The switch-mode converter in CCM is also limited by the current capability of the switch (e.g., switch 108) and diode (e.g., diode 111).
In various instances, transient power produced from a system utilizing a switch-mode converter is, higher than the rated maximum. In a pure DCM system, components must be rated for the peak transient. Thus, it may be desired to allow a system with a switch-mode converter operating in DCM to enter into CCM operation on a temporary basis to allow the system to deliver more power. However, controlling such a system in CCM without current sensing has made for unreliable designs, as the inductor current can easily “run away” and become excessive.
Thus, it is needed and desired to provide a switch-mode converter that can operate in a mode that has the advantages of both DCM and CCM and that minimizes or eliminates at least some of the disadvantages of operating in CCM and/or DCM. It is further needed and desired to provide a way to operate the switch-mode converter in a hybrid DCM/CCM mode and to be able to operate in such a mode such that current sensing is not required. It is additionally needed and desired to be able to operate such a switch-mode converter in a hybrid DCM/CCM that can be used in a PFC system as well as for a number of other applications.